An electrostatic dust precipitator

Abstract

 An electrostatic dust precipitator comprises a separation filter (5) having emitter and collector electrodes (4,6), means (1-3) for maintaining a substantially constant d.c. voltage between the electrodes (4,6) during opera­tion of the precipitator, and pulse-generating means (1-3, 7-11) for generating at a given frequency in the order of 30-500 Hz voltage pulses or voltage pulse trains which are superimposed on the d.c. voltage and which are of short duration in relation to the aforesaid frequency. The pulse­generating means (1-3, 7-11) includes capacitive and in­ductive elements (7,8) which together with the capacitance of the separation filter (5) form an electrical oscillation circuit, and a switching device (9) which is incorporated in said circuit and which is periodically actuatable by a control means (11) for activation of the oscillation circuit (5,7,8). The pulse-generating means (1-3, 7-11) further in­cludes a known voltage-controlled frequency converter (1) which is arranged to generate an a.c. voltage having a frequency which is at least three times higher than the frequency of the voltage pulses or voltage pulse trains, and a singel-phase transformer (1) which is connected down­stream of the frequency converter (1) and followed by a rectifier (3).

Description

[0001] The present invention relates to an electrostatic dust preci­pitator which comprises a separation filter having emitter and collector electrodes, means for maintaining a pre-deter­mined substantially constant d.c. voltage between the elec­trodes during operation of the precipitator, and pulse-­generating means for generating at a given frequency in the order of 30-500 Hz voltage pulses or voltage pulse trains which are superimposed on said d.c. voltage and which are of short duration in relation to said frequency, said pulse-­generating means including capacitive and inductive elements which together with the capacitance of the separation filter form an electrical oscillation circuit, and a switching device which is incorporated in said oscillation circuit and which is periodically actuatable by control means for activation of the oscillation circuit.

[0002] Electrostatic precipitators have long been used in industry for the purpose of cleansing gas of dust entrained therewith, and especially for extracting soot particles from flue gases. The aforedescribed technique of superimposing on the d.c. voltage between the electrodes of the precipitator short voltage pulses or voltage pulse trains at long intermediate pulse or pulse-train intervals in relation to the duration of the pulses or pulse trains has long been applied in the art in order to obtain a more uniform distribution of dust on the precipitation electrodes.

[0003] The supply of voltage to the means for generating the afore­said voltage pulses or voltage pulse trains is normally effec­ted through a tranformer which transforms, for exemple, a 50 Hz, 380 V mains voltage to an a.c. voltage of, for example, 40-200 kV, this voltage then being rectified to form a pulsa­tile d.c. voltage. This pulsatile d.c. voltage can also be used for the normal supply of voltage to the electrodes of the dust precipitator and the magnitude of the voltage is then placed immediately beneath that voltage level which in the continuous operation of the precipitator results in breakdown, in the form of a glow discharge in the separation filter.

[0004] It is desirable to be able to impart an optional shape and amplitude to the superimposed voltage pulses or voltage pulse trains. More specifically, there are often desired voltage peaks of such height that the voltage across the precipitator electrodes briefly reaches a voltage level which lies immedia­tely beneath the voltage at which non-permitted arcing takes place between the precipitator electrodes, even when the vol­tage supply is of short duration.

[0005] The known method of using a conventional transformer for trans­forming mains voltage to a voltage adapted for the voltage supply to the aforesaid pulse-generating or pulse-train generating means, and optionally also to the remainder of the dust precipitator, results in a supply voltage which pulsates at such low frequency that a complete re-charge of the capacitive elements incorporates in said pulse-gene­rating means is not always obtained after a voltage pulse or voltage pulse train, therewith causing the amplitude of the following pulse or pulse train to be lower than the optimal. When generating supenerimposed pulses or pulse trains of rela­tively low frequency, this disadvantage can be overcome, either completely or in part, by stepping up the mains voltage with the aid of a three-phase transformer. A three-­phase transformer, however, has the disadvantage of being expensive and, as with conventional single-phase transfor­mers for transforming a.c. voltage of mains frequency, having a considerable weight.

[0006] Accordingly, the object of the present invention is to provide a novel and useful arrangement with which the aforediscussed disadvantages are avoided to at least a substantial extent.

[0007] This object is achieved in accordance with the present inven­tion in that the pulse-generating means of an electrostatic precipitator of the kind described in the introduction in­cludes a known voltage-controlled frequency converter which is arranged to generate an a.c. voltage.having a frequency which is at least three times higher than the frequency of said voltage pulses or trains of pulses, and a single-phase transformer which is connected downstream of said converter and followed by a rectifier. This arrangement advantageously obviates the need of an expensive three-phase transformer, and the increase in frequency afforded in accordance with the invention substantially decreases the costs for and the weight of the singel-phase transformer coupled downstream of the converter in comparison with a single-phase trans­former for stepping up a.c. voltage of mains frequency.

[0008] The invention will now be described in more detail with re­ference to an embodiment thereof illustrated schematically in the accompanying drawing, in which

Figures 1 is a circuit diagram of an electrostatic dust pre­cipitator according to the invention, and

Figure 2a and 2b illustrate diagrammatically and respectively the voltage across the capacitor and the voltage across the filter when the dust precipitator according to the inven­tion is in operation.

[0009] In figure 1 the reference 1 identifies a schematically illu­strated conventional voltage controlled frequency converter, this converter being supplied, for example, from a three­phase a.c. mains voltage supply, and produces a substantial increase in the frequency of the applied a.c. voltage. For example, the converter 1 can be arranged to generate from an input three-phase voltage of 380 V and a frequency of 50 Hz, a substantially constant single phase a.c. voltage of 380 V and a frequency which lies within the range of 1 - 25 kHz. Connected to the output of the converter 1 is a single-phase transformer 2, in which the output voltage of the converter is increased to a substantially constant a.c. voltage having a voltage value lying within, for example, 40-200 kV. This a.c. voltage is rectified in a rectifier bridge 3 whose positive terminal is connected to earth, whereas its negative terminal, which is held at a substantially negative voltage U_B, via components herein­after described, is connected to the emitter electrode 4 of the separation filter 5 of the illustrated dust precipi­tator. The emitter electrode 4 is housed in a conventional manner in an earthed casing 6 forming the collector elec­trode.

[0010] Connected between the d.c. terminal of the rectifier bridge 3 is a capacitor 7, the one terminal of which is thus earthed and the other terminal of which receives negative voltage from the bridge 3. Connected in series between said other terminal of the capacitor 7 and the emitter electrode 4 is an inductance 8 and a parallel-coupling of a thyristor 9, or thyristor chain, and a diode 10, or diode chain. The thyristor 9, or the thyristors, and the diode 10, or the diodes, are connected in mutually oppositely directed con­ductor directions, more specifically so that the diode, or the diodes, has, or have, a blocking effect in a direction towards the bridge 3, and the thyristor, or the thyristors, has, or have, a blocking effect in a direction towards the filter 5.

[0011] An ignition circuit 11 is adapted to ignite the thyristor 9, or the thyristors, at a predetermied adjustable frequency in the order of 30-500 Hz for a relatively short period of time in relation to the ignition frequency. It will be re­ cognized that the filter 5 can, in the main, be likened to a capacitor in which the capacitor plates are formed by the electrodes 4,6. The capacitor 7, the inductance 8 and the filter 5 thus form an oscillation circuit, so that when the thyristor 9 is ignited and current is flowing therethrough, there is obtained an abrupt increase in the voltage U_F across the filter 5 (increase in the negative potential of the emitter electrode 4) and a decrease in the voltage U_C across the capacitor 7. The circuit will then change to a reverse mode, during which current flows through the diode 10, or the chain of diodes, so that the voltage U_F across the fil­ter again falls abruptly to a level in the proximity of the intended holding voltage U_H and the voltage U_C across the capacitor 7 is again increased to a level in the proximity of the output voltage U_B of the rectifying bridge 3.

[0012] Figures 2a and 2b are diagrams which show the respective variations in time of the capacitor voltage U_C and the filter voltage U_F upon applying to the thyristor 9, or the chain of thyristors, ignition pulses of such short duration that the oscillation circuit effects solely one complete oscillation cycle, and when using the components 1, 2 and 3 also for normal voltage supply to the filter 5. In the diagrams, which illustrate the conditions prevailing when the capacitances of the capacitor 7 and the filter 5 are mutually equal, the desired holding voltage U_H between the elec­trodes and the output voltage U_B of the rectifier bridge 3 are shown in morse lines. As beforementioned, the voltages U_C and U_F are negative. Consequently, upon ignition of the thyristor 9, or the thyristors, current flows swiftly from the filter 5 to the capacitor 7, causing U_C to fall and U_F to raise to a corresponding extent, whereupon the oscillation circuit 5, 7, 8 reverses its mode so that U_C again increases and U_F falls. This reversal is not complete, however, since during each reversal the capacitor 7 delivers voltage to the filter 5. This can be seen from the diagram of figure 2a, in which the voltage curve U_C where it slopes towards the U_B-level subsequent to reversal upwardly repre­sents re-charging of the capacitor 7 with the aid of the components 1, 2, 3 to a voltage corresponding to the output voltage of the bridge 3. In addition the voltage peaks of the filter voltage U_F commence at a lower level than that at which they terminate, and the filter voltage U_F between the voltage peaks falls as a result of the discharge which takes place in the filter 5. If desired, the holding voltage between the electrodes 4, 6 can be maintained with the aid of a separate voltage supply circuit. Furthermore, the thyristor 9, or the thyristors, ca be held ignited over a period of such duration that a plurality of oscillation periods are carried out and therewith a voltage pulse train is generated between the filter electrodes 4, 6.

[0013] The arrangement according to the invention affords conside­rable economic benefits and savings in weight and space. By using a frequency converter in accordance with the in­vention, which converter greatly increases the frequency of the applied voltage, only a relatively small and in­expensive transformer, with a low-weight core, is needed to increase the voltage to a value suitable for filter operation. The arrangement according to the invention also provides for a considerably higher electrical efficiency that that achieved with conventional voltage supply systems for electrostatic dust precipitators. In addition hereto, the use of the arrangement according to the invention also enables the voltage supply to the precipitator electrodes to be controlled continuously with the aid of single and in­expensive means in a manner which is extremely accurate and which reacts quickly to changes in the operating condi­tions which require corrections. The dust precipitator can be powered by a three-phase voltage without requiring the provision of an expensive three-phase transformer. The possi­bility of powering the precipitator with a three-phase voltage is highly beneficial, and is often desired from the technical aspect of the plant.

Claims

1. An electrostatic dust precipitator comprising a separa­tion filter (5) having emitter and collector electrodes (4,6), means (1-3) for maintaining a pre-determined sub­stantially constant d.c. voltage (U_H) between the elec­trodes (4,6) during operation of the precipitator, and pulse-generating means (1-3, 7-11) for generating at a given frequency in the order of 30-500 Hz voltage pulses or voltage pulse trains which are superimposed on said d.c. voltage and which are of short duration in relation to said frequency, said pulse-generating means (1-3, 7-11) including capacitive and inductive elements (7,8) which together with the capacitance (U_F) of the separation fil­ter (5) form an electrical oscillation circuit, and a switching device (9) which is incorporated in said circuit and which is periodically actuatable by a control means (11) for activation of the oscillation circuit (5,7,8), charac­terized in the the pulse-generating means (1-3, 7-11) includes a known voltage-controlled frequency converter (1) which is arranged to generate an a.c. voltage having a frequency which is at least three times higher than the frequency of said voltage pulses or trains of pulses, and a single-­phase transformer (1) which is connected downstream of said converter (1) and followed by a rectifier (3).

2. An electrostatic dust precipitator according to Claim 1, characterized in taht the capacitive and inductive elements incorporated in the oscillation circuit comprise a capacitor (7) which is connected in parallel with the rectifier (3), and a coil (8) connected in series with the electrodes (4,6), between said electrodes and the capacitor (7).

Drawing